The present invention relates generally to variable compression internal combustion engines. More particularly, the invention relates to a method and system for operating a variable compression ratio engine that optimizes fuel economy while and minimizing driver perception of undesirable engine surge and torque variations.
The xe2x80x9ccompression ratioxe2x80x9d of an internal combustion engine is defined as the ratio of the cylinder volume when the piston is at bottom-dead-center (BDC) to the cylinder volume when the piston is at top-dead-center (TDC)xe2x80x94generally, the higher the compression ratio, the higher the thermal efficiency and fuel economy of the internal combustion engine. So-called xe2x80x9cvariable compression ratioxe2x80x9d internal combustion engines have been developed, for example, having higher compression ratios during low load conditions and lower compression ratios during high load conditions.
Because of susceptibility to knock in high load conditions, it is often desirable to operate such engines in a low compression ratio mode when the engine is operating in high load conditions. This however limits fuel economy in a frequently used operating range of the engine. As a compromise, the engine can be operated in a high compression ratio mode while simultaneously retarding spark from maximum brake torque (MBT) so as to avoid knock. The retarded spark however increases variability in indicated torque, which in turn results in an increased amount of engine xe2x80x9csurgexe2x80x9d that may be perceptible to a vehicle operator.
The inventor herein has recognized that since surge can be readily perceived by an operator, especially in relatively steady-state operating conditions, it may be advantageous depending on the amount of surge produced to maintain engine operation at a higher compression ratio with retarded spark during transient operating conditions, e.g., accelerations and decelerations. Accordingly, the inventor herein has disclosed a system and method for modeling engine surge associated with spark retard from MBT. The model can in turn be used to optimally schedule compression ratio mode transitions of a variable compression internal combustion engine.
A method of operating a variable compression ratio internal combustion engine in a motor vehicle is disclosed having the steps of determining a driveline surge based at least in part on operating parameters of the motor vehicle, determining a driveline surge tolerance based at least in part on one or more of the vehicle operating parameters, and selecting one of a plurality of compression ratio operating modes of the engine based at least in part on a comparison of the driveline surge to the driveline surge tolerance. In accordance with a preferred method, the driveline surge is proportional to a baseline engine surge, which itself is proportional to brake engine torque.
In accordance with another preferred method of the present invention, a method of operating a motor vehicle having an a variable compression internal combustion engine includes the steps of determining a brake engine torque, determining a baseline engine surge based at least in part on the brake engine torque, determining a driveline surge based at least in part on the baseline engine surge, determining a driveline surge tolerance based at least in part on one or more operating parameters of the motor vehicle, and scheduling operation of the internal combustion engine in one of the compression ratio operating modes based at least in part of a comparison of the driveline surge to the driveline surge tolerance. With an engine operating in two discrete modes, for example at high compression or low compression, the compression ratio operating mode is selected based on the value of the driveline surge. If the driveline surge exceeds the driveline surge tolerance, then the low compression ratio mode is selected. Otherwise, the high compression mode is selected.
A primary advantage of the above-described methods is that a variable compression internal combustion engine and corresponding motor vehicle can be operated in an optimum compression ratio mode so as to maximize fuel economy while minimizing driver perception of engine surge. By predicting a so-called xe2x80x9cdrivelinexe2x80x9d surge of the vehicle, operation of the engine in the various compression ratio modes can be scheduled so as to minimize driver perception of engine surge. In low speed/high load operating conditions, for example, an engine operating in a high compression ratio, fuel economy mode can remain in such mode depending on the predicted driveline surge. If for example the predicted surge exceeds a certain driveline surge tolerance, then the compression ratio operating mode can be transitioned from the fuel economy mode to a low compression ratio operating mode. In accordance with the present invention, calibratable table values are used to derive the driveline surge from a derived brake engine torque and baseline engine surge.
In a related aspect of the present invention, a corresponding system is also provided for operating a variable compression internal combustion engine in a motor vehicle. The system includes an apparatus for varying the compression ratio of the internal combustion engine and an electronic controller coupled thereto for determining a driveline surge based at least in part on operating parameters of the motor vehicle, determining a driveline surge tolerance based at least in part on one or more of the vehicle operating parameters, selecting one of the compression ratio operating modes based at least in part on a comparison of the driveline surge to the driveline surge tolerance, and operating the variable compression apparatus to configure the internal combustion engine in one of the compression ratio operating modes.
Further advantages, objects and features of the invention will become apparent from the following detailed description of the invention taken in conjunction with the accompanying figures showing illustrative embodiments of the invention.